This invention relates to processes effective to reduce emission of sulfur oxides, e.g., to the atmosphere, and to compositions useful in such processes. In one specific embodiment, the invention involves a process for catalytic cracking sulfur-containing hydrocarbon feedstocks in a manner to effect a reduction in the amount of sulfur oxides emitted from the regeneration zone of a hydrocarbon catalytic cracking unit and to a catalyst composition useful in such catalytic cracking process.
Typically, catalytic cracking of hydrocarbons takes place in a reaction zone at hydrocarbon cracking conditions to produce at least one hydrocarbon product and to cause carbonaceous material (coke) to be deposited on the catalyst. Additionally, some sulfur, originally present in the feed hydrocarbons may also be deposited, e.g., as a component of the coke, on the catalyst. It has been reported that approximately 50% of the feed sulfur is converted to H.sub.2 S in the FCC reactor, 40% remains in the liquid products and about 10% is deposited on the catalyst. These amounts vary with the type of feed, rate of hydrocarbon recycle, steam stripping rate, the type of catalyst, reactor temperature, etc.
Sulfur-containing coke deposits tend to deactivate cracking catalyst. Cracking catalyst is advantageously continuously regenerated, by combustion with oxygen-containing gas in a regeneration zone, to low coke levels, typically below about 0.4% by weight, to perform satisfactorily when it is recycled to the reactor. In the regeneration zone, at least a portion of sulfur, along with carbon and hydrogen, which is deposited on the catalyst, is oxidized and leaves in the form of sulfur oxides, i.e., SO.sub.2, SO.sub.3 and mixtures thereof, along with substantial amounts of CO, CO.sub.2 and H.sub.2 O.
Considerable recent research effort has been directed to the reduction of sulfur oxide emissions from the regeneration zones of hydrocarbon catalytic cracking units. One technique involves circulating one or more metal oxides, capable of associating with oxides of sulfur, with the cracking catalyst inventory in the regeneration zone. When the particles containing associated oxides of sulfur are thereafter circulated to the reducing atmosphere of the cracking zone, the associated sulfur compounds are released as gaseous sulfur-bearing material, such as hydrogen sulfide, which is discharged with the products from the cracking zone in a form which can be readily handled in a typical facility, e.g., a petroleum refinery. The metal oxide reactant is regenerated to an active form, and is capable of further associating with sulfur oxides when cycled to the regeneration zone.
U.S. Pat. No. 4,472,267 presents a summary of the early published work on reducing sulfur oxide emissions from catalytic cracking units.
Perovskites have been disclosed as cracking catalysts. See U.S. Pat. No. 4,055,513. The ideal perovskite crystalline structure is defined by the empirical formula ABO.sub.3 in which A and B are cations of two different metals and in which the A cation is coordinated to twelve oxygen atoms while the B cation occupies octahedral sites and is coordinated to six oxygen atoms. For example, the compound LaMnO.sub.3 has the ideal perovskite structure while other materials, such as La.sub.O.7 Sr.sub.O.3 MnO.sub.3, which exhibit a variety of other structures are still classed as perovskite-type components. U.S. Pat. No. 4,055,513 does not disclose, and is not concerned with, reducing sulfur oxide emissions.
The mineral perovskite (CaTiO.sub.3) possesses a cubic crystal structure at elevated temperatures. An extensive discussion of "PEROVSKITE-RELATED OXIDES AS OXIDATION-REDUCTION CATALYSTS" has been presented by R. J. H. Voorhoeve in Chapter 5 of "Advanced Materials in Catalysis", J. J. Buraton and R. L. Garten, Academic Press (1977). In addition, properties of perovskite-type catalysts have been reported by Nakamura et al. in "Bulletin of Chemical Society of Japan", vol. 55 (1982) at pages 394-399, and in "Journal of Catalysis", vol. 83 (1983) at pages 151-159; and by Happel et al, in "Ind. Eng. Chem. Product Research Development", vol. 14 (1975) at pages 164-168. Catalyst preparation has been discussed by Johnson et al. in "Ceramic Bulletin", vol. 56 (1977) at pages 785-788; and in U.S. Pat. No. 4,055,513. Nakamura et al. investigated oxidation-reduction properties of lanthanum cobaltate, substituted with a minor proportion of strontium, in the oxidation of carbon monoxide and reduction of nitric oxide, as in automotive exhausts, and extended their studies to include oxidation of methanol and propane. Happel et al. employed lanthanum titanate for the reduction of SO.sub.2, to elemental sulfur, with carbon monoxide. Johnson et al. prepared lanthanum manganates, substituted with either strontium or lead, impregnated on ceramic supports for more homogeneous distribution and higher activity for CO oxidation. In U.S. Pat. No. 4,055,513, supported perovskites were prepared in which the support comprised a metal oxide, such as alumina, having a surface coating of a spinel.
Cobaltate perovskites have been suggested as substitutes for noble metals in electro-reactions and as catalysts for use in the oxidation of CO in automotive exhausts as well as for the reduction of NO therein. Such catalysts have found little use in automotive systems because of their deactivation by sulfur oxides. See the work presented by R. J. H. Voorhoeve, noted above.
There continues to be a need for materials used for reducing sulfur oxide atmospheric emissions, in particular from hydrocarbon conversion operations.